1. Field of the Invention
The present invention relates to a damping coefficient control apparatus for a damper device in a suspension mechanism disposed between a sprung mass and an unsprung mass of a wheeled vehicle.
2. Description of the Prior Art
Disclosed in Japanese Patent Laid-open Publication No. 5(1993)-294122 is a control apparatus for controlling a damping coefficient of a damper device in a vehicle suspension mechanism on a basis of Skyhook theory. In the control apparatus, an absolute vertical movement velocity of a sprung mass of a wheeled vehicle and a relative vertical movement velocity of the sprung mass to an unsprung mass of the vehicle are detected to decrease the damping coefficient of the damper device when the absolute vertical movement velocity and relative vertical movement velocity are different in their movement directions and to increase the damping coefficient of the damper device in accordance with an increase of a ratio of the absolute vertical movement velocity to the relative vertical movement velocity when the absolute vertical movement velocity and relative vertical movement velocity coincide in their movement directions.
In such control of the damping coefficient based on the Skyhook theory, it is premised that there is not any restriction in the relative vertical displacement amount of the sprung mass to the unsprung mass. In an actual damper device in the suspension mechanism, however, a rebound stopper is assembled with the piston rod of the damper device to avoid the piston in the damper device from abutment against the internal upper end of a cylinder tube, and a bound stopper is mounted to the vehicle body structure to avoid the cylinder tube against the vehicle body structure. On the other hand, when vertical vibration of the sprung mass is in a vibration region under control based on the Skyhook theory, the damping coefficient of the damper device is maintained in a small value to permit free displacement of the sprung mass relative to the unsprung mass. As a result, when the road wheels of the vehicle fall in a recessed portion on a travel road or ride over a projection on the travel road, free displacement of the sprung mass causes the rebound stopper to abut against the internal upper end of the cylinder tube or the bound stopper to abut against the head portion of the cylinder tube. Although in such an instance, the rebound stopper or bound stopper acts to absorb a great impact acting on the cylinder tube, there occur unpleasant noise and shock.